Add support for alias $x $y

[sampersand]

(Originally a discussion, #1611, I think this is more apt as an issue).

One of the lesser-known facts of ruby is you can also use alias to alias global variables to one another, such as alias $LOAD_PATH $:.

I propose we support this syntax in RBS (it'd be useful for require 'English'). It's very, very niche, but should be supported eventually for completeness.

[sampersand]

Shunt to the back of the backlog?

[soutaro]

I understand we need it for completeness, but I don't think it's a priority because it's niche...

I'm not against implementing it if someone can send a PR. The outline would be:

1. Add a syntax AST::Declarations::GlobalAlias or something?
2. Implement parser
3. Add a method to resolve type of global like Environment#global_type: (Symbol) -> Types::t?
4. Need a validation to reject circular definition